A small-sized motor--which includes a cylindrical, metallic casing and magnets mounted on the inner surface of the casing and serving as stator-side magnetic poles--has been used to drive electric equipment for use in an automobile. Rotor-side magnetic poles of such a small-sized motor are of an odd or even number. For example, as shown in FIG. 8, in the case of a small-sized motor including right-hand and left-hand magnets 1 forming two stator-side magnetic poles, and a 5-pole, laminated core 8 forming rotor-side magnetic poles of an odd number, when one rotor-side magnetic pole is positioned on the X axis, the five rotor-side magnetic poles become asymmetrical with respect to the Y axis. However, when, as a result of rotational progress, one rotor-side magnetic pole comes to be positioned on the Y axis, the rotor-side magnetic poles become symmetrical with respect to the Y axis, as shown in FIG. 9. Repeatedly becoming symmetrical and asymmetrical according to rotation as viewed from the stationary-side magnetic poles, a rotor having an odd number of poles is not advantageous in terms of vibration.
By contrast, when two magnet-type magnetic poles are combined with a rotor having an even number of poles, for example, 6 poles, the rotor becomes symmetrical in any rotational position. Thus, the rotor having an even number of poles is advantageous in terms of vibration. However, when a small-sized motor employing a rotor having an odd number of poles and that employing a rotor having an even number of poles are compared under substantially the same conditions (motor size, voltage, and current, among others), the rotor having an odd number of poles is conventionally known to exhibit a larger torque. The small-sized motor employing a rotor having an even number of poles may raise a cogging problem.
A motor employing a rotor having an odd number of poles is advantageously applicable to fields requiring a large torque, and, at the same time, is required to suppress an inherent vibration problem. Conventionally, the profile of a laminated core has not been considered from the viewpoint of suppression of vibration. Specifically, the profile of the conventional laminated core shown in FIGS. 8 and 9; i.e., the width of arm portions 16, the thickness t of wing portions 17, and the distance between the adjacent wing portions 17, constituting salient magnetic poles of the rotor have not been considered from the viewpoint of suppression of vibration.